Phenol resin is excellent in mechanical characteristics, electric characteristics, heat resistance, and adhesiveness, and is used as a binder for friction material such as a disk pad or drum brake lining.
A non-asbestos type disk pad in which asbestos is not used due to the problem of carcinogenicity has been widely used in Japan, Korea, and North America. However, in some cases, there are problems relating to squeal and judder (NVH: noise, vibration, and harsh).
In particular, a phenomenon called morning squeal, which is a large squeal or vibration generated during first few times of braking after a car starts from a garage in the morning in winter when the temperature is low, is regarded as a problem. In the morning in winter when the temperature is low, the braking when a driver lightly steps on a brake causes an urgent brake, and as a result, a sound or a vibration is generated in some cases.
There are cases where the same case as above occurs even after parking a vehicle for a long period of time in the rain or even in an area with a high humidity, not only in the morning.
It is almost impossible for the system to deal with such a problem. Therefore, it is necessary to provide resin, which is used in the disk pad, with low hygroscopicity and high vibration absorbency for absorbing or reducing the squeal and the judder.
In order to improve the characteristics of the phenol resin, various studies in modified phenol resins such as elastomer-modified phenol resins, oil-modified phenol resins, cashew-modified phenol resins, silicone-modified phenol resins, epoxy-modified phenol resins, and melamine-modified phenol resins.
Patent Document 1 (Japanese Unexamined Patent Publication No. 60-184533) discloses technology of mixing acrylonitrile-butadiene rubber (NBR) with the phenol-based resin in order to improve impact resistance and flexibility. However, the particle diameter of NBR is greater than 200 μm and NBR has poor compatibility with the phenol-based resin. For this reason, NBR has poor dispersibility in the phenol resin, and therefore, it is necessary to mix a large quantity of NBR in order to improve the impact resistance and the flexibility. When the mixing quantity of NBR is large, heat resistance deteriorates and an interface between phenol resin phase and NBR phase is separated.
In addition, there is technology of using elastomer-modified phenol resins with carboxy-terminal liquid NBR in order to improve the problem of the dispersibility. When curing of the phenol resin progresses using a curing agent, toughness is added to the phenol resin due to the elastomer, which is dissolved therein, is phase-separated into a spherical shape. However, due to the use of the phase separation during the curing, the curing of the phenol resin tends to be greatly affected by curing conditions such as the type of curing agents, the temperature, and time, and thus, it is hard to obtain stable characteristics of cured material.
Patent Document 2 (Japanese Unexamined Patent Publication No. 59-6246) discloses a thermosetting resin composition through which it is possible to obtain a cured material excellent in heat resistance, flexibility, and dimensional stability and which is formed of a copolymer having isopropenylphenol as a component, and phenol resin. However, there is no specific examination relating to the resin composition providing a cured material excellent in wear resistance and vibration absorbency.
Patent Document 3 (Japanese Unexamined Patent Publication No. 2006-265321) discloses a phenol resin molding material containing phenol resin and high molecular weight polyethylene, as a molded product excellent in mechanical strength and wear resistance. Since the phenol resin molding material contains the high molecular weight polyethylene, fine matrix is formed on a surface. As a result, the hardness becomes hard, thereby improving the wear characteristics.